Unlike poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT) becomes friable after being crystallized and, especially, after being further polymerized in the solid state (SSP). Because of its friability, during conveying and shipping of crystallized or solid stated PTT, which is usually in pellet form, excessive amounts of dust and fines are generated, incurring a substantial material loss and creating problems during subsequent processing. We have discovered three unexpected ways to increase the robustness of crystallized and solid stated PTT pellets, thereby minimizing dust and fines generation problems during handling of such PTT pellets.
PTT is a newly commercialized polyester whose properties and processes for manufacture are similar to those of the well-known and most widely used polyester, PET. PTT possesses a unique combination of properties suitable for fiber and engineering plastic applications. For fiber applications, the required intrinsic viscosity (IV) of PTT is between 0.80 and 0.94 dl/g (equivalent to number average molecular weights of 18,000 and 20,000 respectively). This is approximately the same number average molecular weight range for PET used in textile fiber applications. PTT for fiber applications can be advantageously produced by a combination of a melt polymerization process and a solid state polymerization (SSP) process because of the reasons given below.
Because PTT is substantially less stable than PET and hence more susceptible to thermal degradation than PET in melt state, the melt polycondensation of PTT should be conducted at temperatures at least 30xc2x0 C. below that for PET. Furthermore, because the major polycondensation by-product of PTT, 1,3-propanediol (PDO), is substantially less volatile than that of PET, ethylene glycol (EG), a thin-film type polycondensation reactor, such as a disk-ring reactor, should be used to effectively remove the polycondensation by-products to achieve the IV required for fiber-grade PTT. Consequently, a polycondensation time several times longer than for PET and a disk-ring type polycondensation reactor several times larger than for PET should be used for PTT. It is a very expensive process if a melt polymerization process alone is used to produce PTT for fiber applications. Furthermore, even at such lower melt polycondensation temperatures, the long residence time required to achieve the desired IV will result in inferior product properties, especially color. By terminating melt polycondensation earlier, to limit thermal degradation, and further polymerizing the melt polycondensation product in solid state at a much lower temperature to the IV suitable for the desired application, better overall process economics and superior product quality, especially in terms of color, can be achieved.
When the desired IV is reached, the melt polycondensation product is usually extruded through a strand die to produce melt strands, which are quenched with water (quenching water) to solidify them, which are then chopped into pellets with a pelletizer. The PTT pellets thus obtained may be used directly for fiber spinning, if the IV is sufficiently high, or otherwise used as a prepolymer for further polymerization in solid state.
In the pelletizing system for polyesters, including PET and PTT, deionized water (DI water) or soft water is used as the quenching water to protect the equipment, especially the cutter. The spent DI water, which is warm, is recycled after being cooled in a heat exchanger usually with utility water, which, in turn, is usually cooled in a cooling tower. Therefore, the temperature of the quenching water is usually somewhat higher than room temperature (i.e., 77xc2x0 F. or 25xc2x0 C.). It is not unusual that the quenching water temperature gets as high as 104xc2x0 F. (40xc2x0 C.) during the summer time.
Because of the relatively low Tg (about 45xc2x0 C.) and relatively high crystallization rate, PTT pellets thus obtained have a crystallinity of between 10 and 20 wt. % (vs. 3-5 wt. % for PET pellets). Nevertheless, for convenience, the PTT pellets thus obtained are referred to as xe2x80x9camorphousxe2x80x9d pellets.
Crystallization of the pelletized amorphous PTT is required for the following reasons:
1. To prevent blocking of amorphous pellets during shipping or storage in summer months.
2. To prevent sticking or agglomeration of pellets during drying prior to spinning.
3. To prevent sticking of pellets during solid state polymerization.
In the summer months, the interior temperature of a rail car or an exposed storage silo can reach as high as 140xc2x0 F. (60xc2x0 C.), exceeding the Tg of PTT. Under these shipping and storage conditions, amorphous PTT pellets become tacky and stick together to form blocks. To prevent blocking of PTT pellets during shipping or storage in the summer months, PTT pellets must be crystallized to achieve a crystallinity of at least 36%. Crystallization of PTT can be most efficiently conducted at temperatures between 140 and 170xc2x0 C. Because sticking of pellets occurs before the pellets develop a sufficient crystallinity, crystallization is usually conducted under vigorous agitation in a mechanically agitated vessel, a tumble vessel, or a fluidized bed. The typical crystallization time is between 10 minutes and one hour, depending on the type of crystallizer used.
To minimize hydrolytic degradation of PTT during melt processing, such as spinning, PTT must be dried to a moisture content below 0.005% beforehand, usually also at temperatures between 140 and 170xc2x0 C. To prevent sticking during drying, amorphous PTT pellets must be precrystallized, usually at the same temperature used for drying. Once PTT pellets are sufficiently crystallized, they can be dried in a moving bed or a drying hopper without sticking. Therefore crystallization and drying of PTT can be conducted in the same vessel or in two different vessels. Although, in large-scale operations, crystallization and drying of PTT are most economically conducted in separate vessels, at many fiber-spinning facilities, tumble dryers are used to crystallize and dry polyesters.
SSP of PTT is effected at 190xc2x0 C. or higher temperatures. To prevent PTT pellets from sticking together or to the reactor wall, amorphous PTT prepolymer must be crystallized at an early stage of the SSP process.
We have found that PTT pellets produced by conventional process, unlike PET pellets, become brittle or friable after undergoing crystallization or SSP. Because of the friability of the pellets, excessive amounts of dust and fines can be generated during shipping and conveying of crystallized or solid stated PTT products, incurring a substantial material loss and creating problems in the downstream operations. The most pronounced dust and fines generation occurs in high-speed pneumatic conveying systems, such as a dilute phase conveying system. Material losses as high as 15% through dust and fines generation during conveying of solid stated PTT have been reported.
We have unexpectedly discovered that the friability of crystallized PTT and solid stated PTT can be effectively reduced by lowering the temperature of the quenching water used in the pelletizing of the melt polycondensation polymer to between 32xc2x0 F. (0xc2x0 C.) and 65xc2x0 F. (18xc2x0 C.). In addition, we also discovered that the robustness of solid stated PTT pellets is effectively increased by using a prepolymer with a lower IV or by increasing the IV of the solid stated product. The normal range for prepolymer IV is 0.60 to 0.70 dl/g. In this invention, it should be 0.35 to 0.70 dl/g. The normal range for SSP IV is 0.80 to 0.94 dl/g. For this invention, it should be 0.80 to 2.00 dl/g.